1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a transfer unit used therein.
2. Description of the Background
Image forming apparatuses are used as copiers, facsimile machines, printers, or multi-functional devices thereof. Conventionally, various types of color image forming apparatuses have been proposed. For example, one type of color image forming apparatus employs a direct transfer method in which toner images formed on a plurality of image carriers are directly and collectively transferred onto a recording medium. Alternatively, another type of color image forming apparatus employs an intermediate or indirect transfer method in which toner images are primarily and collectively transferred onto an intermediate transfer member and then transferred onto a recording medium.
In either type, such electrophotographic color image forming apparatus typically charges each image carrier by a charging unit of an image forming unit and emits a light beam from a light source, for example, a laser diode (LD) or light-emitting diode (LED), to write an electrostatic latent image on the surface of each image carrier. Then, such electrophotographic image forming apparatus visualizes the latent image by a developing unit to from a toner image on the surface of each image carrier.
Further, one type of color image forming apparatus employing an intermediate transfer method has a plurality of image forming units that contact an intermediate transfer member, serving as a transfer member, at different positions. The intermediate transfer member may be, for example, an endless-shaped intermediate transfer belt extending over a plurality of rollers.
Such image forming apparatus has a plurality of primary transfer units corresponding to the image forming units. Each primary transfer unit transfers a toner image, formed on each image carrier, onto the intermediate transfer belt. Specifically, in each primary transfer unit, a primary transfer area is formed between each image carrier and the intermediate transfer belt. By action of transfer electric field generated at each primary transfer area, the toner image on each image carrier is transferred onto the intermediate transfer belt.
When using such intermediate transfer member, such image forming apparatus has a secondary transfer unit with which the toner images on the intermediate transfer member are transferred onto a recoding medium such as a paper sheet. Specifically, a transfer electric field is generated at a secondary transfer area between the intermediate transfer belt and the recording medium. By action of such transfer electric field, the toner images on the intermediate transfer belt are transferred onto the recording medium.
The electrostatic latent images formed on the respective image carriers are developed with charged toners of different colors. At the primary transfer area at which each image carrier and the intermediate transfer belt contacts and faces each other, typically a transfer bias is applied to the intermediate transfer member, thereby generating a transfer electric field. By action of such electric field, the toner images on the image carriers are transferred in turn onto the intermediate transfer member to form a color image.
Such transfer units need to transfer the toner images onto the intermediate transfer member or recording medium so that its original image is precisely and stably reproduced before and after the transfer process. In other words, to achieve a performance level suitable for such primary and secondary transfer units, a transfer process needs to be stably conducted with a relatively high transfer efficiency.
Such color image forming apparatuses may have a charging member using a corona charging method or a charging member using a contact charging method. One example of corona charging member is an electrifying charger, and one example of contact charging member is a charging roller.
In a corona charging method, a charging member may have discharge electrodes, such as wire electrodes, and shield electrodes surrounding the discharge electrodes. Such corona charging member applies high voltages to the discharge electrodes and shield electrodes to generate a corona shower, and charges the surface of a charged body, such as an image carrier, by the corona shower to a certain electric potential. However, such corona charging method may generate a relatively large amount of ozone and/or may need a relatively high voltage.
In this regard, recent years certain types of contact charging methods have come into practical use because of advantages such as a relatively low ozone generation rate and electric consumption compared to the corona charging method. For one contact charging method, a charging bias is applied to a charging member in contact with a charged body, so that a surface of the charged body is charged to a certain potential. Such contact charging method may be performed by a charging member of, for example, roller-type, fur-brush-type, magnetic-brush-type, or blade-type.
For one roller-type charging member (hereinafter “charging roller”), direct-current (DC) bias and alternating-current (AC) bias are superposed one on the other to be applied to the charging roller, so that the surface of the charging member is uniformly charged to a certain potential. However, for such charging roller, the application of AC bias may result in a larger discharge amount than the above-described corona charging member, thereby resulting in damage to an image carrier or photoconductor, for example, curling or roughness of the surface of photoconductor.
To prevent such damage, lubricant may be applied to the surface of photoconductor. Such lubricant may prevent the curling of the surface of photoconductor, although a portion of lubricant may be fixed to the charging roller, thereby inhibiting the surface of photoconductor from being uniformly charged.
Accordingly, optimization has been attempted to obtain an application amount of lubricant compatible for both the curling of the surface of photoconductor and the adhesion of lubricant to the surface of photoconductor. However, it is quite difficult to find a completely-compatible application amount for both factors, and thus the service life of charging roller may be put second.
The above-described corona charging method is a non-contact charging method. Such non-contact charging method can relatively suppress deterioration of a charging unit due to lubricant or toner, thereby suppressing damages to a photoconductor. Accordingly, to prevent damages to the photoconductor, a sufficient amount of lubricant can be applied to the surface of photoconductor with little consideration of contamination of such lubricant or toner to the charging unit.
Thus, the corona charging member may have disadvantages in ozone generation amount and electric consumption compared to the charging roller. By contrast, the corona charging member may have advantages in service life compared to the charging roller.
As another type of charging method, a proximate charging method has been proposed in which a charging roller is disposed proximate to and in non-contact with a photoconductor. Such configuration may prevent a reduction in charging performance due to foreign matter attached to the photoconductor, for example, while suppressing the generation amount of ozone by utilizing a charging property similar to that of the contact charging method.
In consideration of such characteristic of each charging method, one type of conventional image forming apparatus has a plurality of toner-image forming units each including any one of the electrifying charger and the charging roller according to toner color. For example, such electrifying charger, which has a relatively long service life, may be used in a frequently-used image forming unit of black color while such charging roller, which has a relatively low ozone generation rate and electric consumption, may be used in a less-frequently-used image forming unit of a color other than black. Such configuration can reduce the frequency of maintenance operations in the image forming apparatus, thereby facilitating a reduction in the generation amount of ozone and electric consumption, which are increasingly demanded from a viewpoint of environmental concern.
Such conventional image forming apparatus may also have a plurality of pressing units that press the intermediate transfer member to the surfaces of image carriers at respective primary transfer positions. Applying such pressure to a transfer area between each image carrier and the intermediate transfer member during the primary transfer process can enhance transfer efficiency, thereby preventing occurrences of transfer failures such as white dropout in a transferred image.
Accordingly, using such pressing units can suppress waving of the intermediate transfer member at each transfer position. As a result, the intermediate transfer member can uniformly contact the surface of each image carrier, thereby suppressing transfer irregularity.
However, when pressing the transfer area between the intermediate transfer member and each image carrier, stress may be concentrated on a portion of the toner image formed on the intermediate transfer member, thereby resulting in partial dropout of toner image during the transfer process (hereinafter “image dropout”). Such image dropout during the transfer process may notably appear when a relatively large amount of toner is attached to the intermediate transfer unit as in the case where multi-color images are superimposed one on another.
To prevent such image dropout, one type of conventional image forming apparatus sets a contacting pressure of a pressing unit within a certain range. Alternatively, for another type of conventional image forming apparatus, a contacting pressure at a transfer area on a downstream side in a sheet transfer direction thereof is set lower than a contacting pressure at a transfer area on an upstream side.
Still another type of conventional image forming apparatus employs different contacting pressures between a transfer nip of black toner and a transfer area on the uppermost stream. Still another type of conventional image forming apparatus is a tandem-type image forming apparatus that includes a corona charging member and a contact charging member.
However, for such conventional image forming apparatus including a corona charging member and a contact charging member, shortage of transfer efficiency or image dropout during the transfer process may be generated. Alternatively, in such conventional image forming apparatus employing an intermediate transfer member, when a toner image is secondarily transferred onto a recording medium, such as a paper sheet, of low smoothness, a transfer performance may vary due to irregularity of the surface of recording medium. As a result, image quality may be degraded, thereby resulting in surface roughness or image-density irregularity of a resultant image.
Consequently, there is still a need for an image forming apparatus including a transfer unit capable of effectively suppressing failures such as shortage of transfer efficiency, image dropout during the transfer process, and patchy irregularity of image-density.